1. Field of the Invention
The present invention relates generally to a distributed feedback semiconductor laser. More specifically, the invention relates to a distributed feedback semiconductor laser which can oscillate in a single longitudinal mode.
2. Description of the Prior Art
In a distributed feedback semiconductor laser, oscillation in a single longitudinal mode is desirable. Conventional distributed feedback semiconductor lasers have used gratings which are arranged in a regular pitch for distributively feeding the light back by Bragg reflection. However, it has been difficult to generate a single longitudinal mode in conventional distributed feedback semiconductor lasers, since often two longitudinal modes of light are generated thereby producing a mode-hopping.
In order to eliminate the aforementioned disadvantage and to achieve a single longitudinal mode, distributed feedback semiconductor lasers have been proposed which can change pitch near the center of gratings for generating a .pi. phase difference, that is, .lambda./4 phase shift where .lambda. is the oscillation wavelength.
However, it is difficult to manufacture an accurate grating which can change pitch near the center gratings. Thus there is a possibility of manufacturing a large quantity of inferior goods thus lowering the yield of lasers manufactured.